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Chiller Plant Water-Side Economizer Commissioning

SOP Process Updated Jul 24, 2026 ยท HVAC

Why this matters

A water-side economizer (WSE) lets a chiller plant make chilled water from cooling-tower water alone when outdoor wet-bulb drops, killing compressor kWh during shoulder seasons and winter. ASHRAE 90.1-2022 Section 6.5.1.2 requires economizers on most chilled-water systems above plant capacity thresholds, and the operating engineer has to prove the sequence works at acceptance. A commissioning miss here means the chiller never unloads at 45 F wet-bulb, the building owner pays for compressor work the plant should never have done, and the M+V data fails LEED EAp2 or local energy-code post-occupancy verification.

System types you will see

Two architectures dominate. Plate-and-frame HX in series with the chiller evaporator: tower water passes through the HX on the condenser side, chilled-water loop passes through the HX on the load side, the chiller is bypassed when supply temp can be met. Integrated-strainer cycle (less common, older installations): tower water is dosed directly into the chilled-water loop through a filter. Most new plants use the plate-and-frame approach because it isolates open cooling-tower water from the closed chilled-water loop.

Pre-commissioning checklist

Verify the HX plate count and design GPM match the schedule. Confirm condenser-water bypass valve and chilled-water bypass valve are installed in the correct flow paths, not reversed. Check that tower-water supply temperature sensor is in the supply line ahead of the HX, not after. Confirm the BAS is reading wet-bulb from a real outdoor sensor; many plants ship with the dry-bulb point mapped because the sensor was a late add. Pull cooling-tower fan and bypass dampers through manual command to verify they stroke full open and full closed. Walk the chilled-water flow path with valve schedule in hand; a reversed three-way is the single most common commissioning miss.

Changeover sequence to verify

Drive the plant from mechanical to WSE by manually lowering the outdoor wet-bulb setpoint or simulating with a signal generator. Watch the sequence:

  1. Tower fans ramp to maintain condenser-water supply at WSE setpoint (typically design CHWS minus the HX approach, often 38 to 42 F when CHWS is 44 F and HX approach is 4 to 6 F).
  2. Condenser-water bypass closes around the chiller, routing flow to the HX.
  3. Chilled-water bypass opens around the chiller evaporator.
  4. Chiller compressor signals an unload-and-stop ramp.
  5. Once chiller is off and isolation valves close, plant is in full WSE.

Reverse the sequence on rising wet-bulb. Confirm there is no oscillation at the changeover band; ASHRAE Guideline 36-2021 recommends a deadband of at least 2 F on the wet-bulb crossover and a minimum 10-minute timer between mode changes.

Performance verification

Trend CHWS temp, CHWR temp, plant kW, and outdoor wet-bulb for at least one full week spanning a wet-bulb range below the changeover. Compute plant kW/ton on both sides of the changeover; the WSE side should drop to compressor-house parasitic loads only (pumps, fans, controls), typically 0.10 to 0.20 kW/ton compared to 0.55 to 0.70 kW/ton on mechanical cooling. If the plant cannot make setpoint at the design wet-bulb, check HX fouling (clean side of plates with CIP), tower fan VFD top speed (often field-set below schedule), and CHW flow through the HX (balancing valve may be partly shut).

Documentation deliverables

Hand the owner a commissioning report containing: sequence-of-operations marked up against the as-built BAS programming, trended data showing successful changeover events, kW/ton plot before and after WSE engagement, list of setpoints loaded into the BAS (wet-bulb enable, wet-bulb disable, HX approach, minimum tower-water supply, changeover timers), and a one-page operator guide for manual override. ASHRAE Standard 202-2018 calls this the Systems Manual, and most commissioning agents will not sign acceptance without it.

References

  • ASHRAE Standard 90.1-2022, Section 6.5.1.2 Economizers, and Section 6.5.4 Chilled-Water and Heat-Rejection Systems
  • ASHRAE Guideline 36-2021, High-Performance Sequences of Operation for HVAC Systems, Section 5.20 Chilled-Water Plants
  • ASHRAE Standard 202-2018, Commissioning Process for Buildings and Systems
  • Carrier System Design Manual Part 5, Water Conditioning and Cooling Tower Theory
  • Trane Engineers Newsletter ENEWS-39-2 Water-Side Economizing

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